Semiconductor memory devices often include memory devices fabricated with semiconductor materials such as, for example, silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP). Such semiconductor memory devices may be further classified into volatile memory devices and nonvolatile memory devices.
The volatile memory devices include memory devices that are designed to lose internally stored data upon a power supply interruption. The volatile memory devices may include, for example, a static random access memory (SRAM), a dynamic RAM (DRAM), and a synchronous DRAM (SDRAM). In contrast, the nonvolatile memory devices include memory devices that retain stored data even upon experiencing a power supply interruption. The nonvolatile memory devices may include, for example, a read only memory (ROM), a programmable ROM (PROM), an erasable programmable ROM (EPROM), an electrically erasable programmable ROM (EEPROM), a flash memory, a phase-change RAM (PRAM), a magnetic RAM (MRAM), a resistive RAM (RRAM), and a ferroelectric RAM (FRAM). Both of the volatile and non-volatile flash memory devices as discussed here may be further classified into a NOR type and a NAND type.
Additionally, the flash memory devices may be further classified into 2-dimensional semiconductor devices having strings that are horizontally formed on a semiconductor substrate and 3-dimensional semiconductor devices having strings that are vertically formed on a semiconductor substrate.
Such 3-dimensional semiconductor devices as described here have been designed to address limitations generally involving a degree of integration of the 2-dimensional semiconductor devices by vertically forming a plurality of strings on a substrate. The strings may include, for example, a drain select transistor, memory cells, and a source select transistor, which may be connected in series between a bit line and a source line.